Operation of lean burn engines, for example, diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Additionally, diesel engines offer significant advantages over gasoline (spark ignition) engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present more severe problems than their spark-ignition counterparts. Because diesel engine exhaust gas is a heterogeneous mixture, emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO).
NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO is of concern because it transforms into NO2 in the upper atmosphere where it is believed to undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons, and is a significant contributor to acid rain. Ground level NO2, on the other hand, has a high potential as an oxidant and is a strong lung irritant.
Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions. One of these strategies utilizes selective catalytic reduction (SCR) of NOx, which involves the reaction of NOx in the presence of a reductant (e.g. urea) over a SCR catalyst, for example vanadia-titania based catalysts or zeolites promoted with a base metal such as Cu, Fe, or other base metals. A performance enhancement can be observed when there is an adequate ratio of NO2/NOx in the feed gas to the SCR catalyst, especially in the low temperature range (i.e. <250° C.). Oxidation catalysts comprising a precious metal such as a platinum group metal (PGM) dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), which are placed in the exhaust flow path from a diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic carrier substrates (such as, e.g. a flow-through monolith carrier), upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO, and the soluble organic fraction (SOF) of particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation because the engine exhaust is not at a temperature sufficiently high enough for efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to include an adsorbent material, such as a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature.
Oxidation catalysts comprising a platinum group metal (PGM) dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum (Pt) remains the most effective metal for oxidizing CO and HC in a DOC, after high temperature aging under lean conditions and in the presence of fuel sulfur. One of the major advantages of using palladium (Pd) based catalysts is the lower cost of Pd compared to Pt. However, Pd based diesel oxidation catalysts typically show higher light-off temperatures for oxidation of CO and HC, especially when used to treat exhaust containing high levels of sulfur (from high sulfur containing fuels) or when used with HC storage materials. The “light-off” temperature for a specific component is the temperature at which 50% of that component reacts. Pd-containing DOCs may poison the activity of Pt to convert HCs and/or oxidize NOx and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically limited the use of Pd-rich oxidation catalysts in lean burn operations, especially for light duty diesel application where engine temperatures remain below 250° C. for most driving conditions.
There is an ongoing need to provided improved diesel oxidation catalysts. It would be desirable to provide a diesel oxidation catalyst (DOC) that provides enhanced NO2 content of the exhaust gas exiting the DOC. An enhanced NO2 content is desirable to improve downstream NOx removal, particularly the performance of downstream SCR catalysts. In addition, it would be desirable to provide a diesel oxidation catalyst that lowers the light off temperature of CO.